Digital cross-connect systems are an integral part of telecommunications transport network. They are increasingly used by all service providers including exchange carriers, long distance carriers, and competitive bypass carriers. Existing digital cross-connect system architectures generally have been based on a single core approach where all cross-connections are made through a single switching node or fabric. To handle layered signal structures used in today's transport networks, these single switching nodes have been connected in series.
Because new data, voice and imaging applications are causing a fundamental shift in the nature of network traffic, the network architecture is required to evolve to accommodate this change. Instead of being dominated by voice data, as in the past, the network traffic will increasingly carry bursty high-speed data transmissions. User applications and new network technologies including frame relay, switched multi-megabit data service and asynchronous transfer mode (ATM) are driving the transport network toward the synchronous optical network or SONET. SONET is a new transport medium, designed to enable midspan meets between central office switching systems. It defines optical signals and a synchronous frame structure for multiplexed traffic as well as for operations and maintenance procedures.
SONET brings a multi-dimensional increase in network complexities. There is a wide variety of signal formats that are embedded in new broadband and wideband structures such as synchronous payload envelopes (SPEs). DS1 signals provide the primary transport rate for North America. DS1 frames are capable of carrying twenty-four DS0 (64 kbs) voice or data channels. DS1 signals can be mapped in the new SONET STS-1 SPEs in a number of ways. 1) The DS1 signals can be multiplexed into DS3 frames via M1/3 multiplexers and the DS3 signals can be asynchronously mapped into the STS-1 SPE. 2) The DS1 signals can be synchronously or asynchronously mapped into floating VT1.5 payloads and the VT1.5 signals can be multiplexed in the STS-1 SPE. 3) The DS1 signals can be mapped into Locked VT1.5 payloads and the Locked VT1.5 signals can be multiplexed into the STS-1 SPE. However, these approaches create three incompatible wideband structures, which must be individually groomed, multiplexed and switched to assure end-to-end signal integrity. This analysis brings to light the fact that networks can no longer deliver traffic transparently. Because the networks have to recognize different payloads to deliver traffic intact between users, the digital cross-connect system must be able to handle all three formats equally well.
Accordingly, advantages have been recognized for a digital cross-connect system that integrates narrowband, wideband and broadband subsystems to route and manipulate circuit as well as cell-based traffic. To accomplish this task, a unique timing architecture is realized to accommodate a distributed hardware architecture that employs separate timing reference signals and to achieve frequency justification and phase alignment of data signals at certain timing interfaces.